The present invention relates to radio communications technology and more particularly to direct conversion radio receivers.
At the present time the vast majority of radio receivers are of the superheterodyne type employing one or more intermediate frequency stages which allow for filtering and amplification at fixed frequencies. Alternatives have always existed to the superheterodyne architecture such as superregenerative and direct conversion designs. However, these alternative designs have been subject to serious flaws which have relegated radio receivers of these types to specialty roles within the radio communications world.
Despite the widespread adoption of the superheterodyne design, it has been widely recognized that the direct conversion architecture holds great promise for superior Performance. For example, direct conversion receivers are not subject to image rejection problems and are not affected by cross-over spurious responses which are so often the cause of interference and related design difficulties in superheterodyne receivers. Further, direct conversion receivers feature simpler low pass type filters operating at audio frequencies in contrast to the often bulky and expensive bandpass filters employed in superheterodyne receivers, require only a single injection signal at one frequency rather than multiple signals at different frequencies (multiple conversion sets), and provide a good potential for VLSI implementations since a majority of the receiver components consist of active audio and digital circuitry.
In a typical I/Q direct conversion receiver incoming RF communications signals are split into a pair of equal components which are in phase with each other. These RF components are then mixed with separate injection signals on frequency with the communications signal but separated by 90.degree. in phase. I and Q baseband component signals which are in quadrature are thereby formed which may be independently filtered and amplified at audio frequencies on a pair of separate signal channels. The I and Q components formed as a result of the mixing process allow the signal to be conveniently and accurately demodulated upon being supplied to a suitable signal processing unit.
This architecture works well except that variations between the signal channels which commonly occur as a result of changes in temperature, frequency and other operational parameters result in gain and phase mismatches which produce distortion products in the output of the receiver. Gain mismatches of as little as 0.2 dB and phase mismatches of as little as 1.degree. can result in 40 dB distortion products. Phase mismatches are especially difficult to regulate and constitute a major design problem inherent in this architecture. The resulting distortion products can not ordinarily be reduced to less than 30-40 dB in practice and correspond to discrete tones which greatly limit the performance of the receiver.
Researchers investigating the design of direct conversion radio receivers have frequently recognized this limitation and a number of systems for correcting for errors between quadrature signal channels have been proposed. However, these systems have in general been specialized designs limited to the processing of signals of only a single modulation type. For example, U.S. Pat. No. 4,926,433 to Werner Reich entitled "Correction Circuit For A Digital Quadrature-Signal Pair" describes a correction system including an error-detecting stage for deriving amplitude, offset and phase errors from which correction signals are formed. However, error detection is limited to wideband FM signals characterized by quadrature signal pairs capable of forming an "elliptical locus" from which the errors can be determined by comparison with an ideal circle. In contrast, AM signals would result in such a locus taking on irregular shapes from which Proper error signals could not be derived.
The publication by Bolliger and Vollenweider entitled "Some Experiments on Direct Conversion Receivers" in the proceedings of the Fifth International Conference on Radio Receivers and Associated Systems held Jul. 24-26, 1990 at Oxford, England describes a useful method for reducing the gain and phase errors between signal channels in an I/Q direct conversion receiver. In accordance with this method, new signals are formed which are equal to 2IQ and I.sup.2 -Q.sup.2 and these signals are highpass filtered to remove their DC components in order to produce new signals I.sub.N and Q.sub.N which are related to twice the phase angle .theta. defined by the original I and Q components and which are characterized by reduced gain and phase errors. The rationale for forming these signals may be readily understood with reference to the basic trigonometric identities cos 2.theta.=cos.sup.2 .theta.-sin.sup.2 .theta. and sin 2.theta.=2 cos .theta. sin .theta.. Since the signals I and Q are in quadrature, they may be seen to correspond to cos .theta. and sin .theta.. Consequently, the expressions 2IQ and I.sup.2 -Q.sup.2 are related to 2.theta. which allows the phase angle to be determined using the I.sub.N and Q.sub.N signals.
In the expressions 2IQ and I.sup.2 -Q.sup.2 phase and gain errors resulting from hardware mismatches between the signal channels are primarily relegated to DC terms which can be filtered out of the signal in order to reduce their effects. The fact that I.sub.N and Q.sub.N relate to 2.theta. rather than .theta. can be compensated for by accordingly adjusting computed phase angles. This method of reducing gain and phase errors between signal channels is highly useful but is limited to angle modulated signals since amplitude modulation would result in the DC terms being converted to low frequency AC components resistant to filtering. Furthermore, this method does not provide for detection of the actual phase and gain errors and full and accurate correction for such errors on an ongoing basis.
It is, therefore, an object of the present invention to provide an I/Q direct conversion receiver which is characterized by superior performance due to the absence of distortion products arising from gain and phase errors between the signal channels.
It is another object of the present invention to provide a system for use in an I/Q direct conversion receiver which is adapted for processing signals of all modulation types and automatically detecting phase and gain errors resulting from mismatches between the signal channels within the receiver and fully correcting for such errors pursuant to a straightforward signal processing algorithm.
It is a yet further object of the present invention to provide a new system for controlling phase and gain errors in an I/Q direct conversion radio receiver which is economic to construct, provides superior performance and may be substantially implemented in VLSI.